The present invention relates to the processing of a C3 to C6 hydrocarbon cut from a cracking process, such as steam or fluid catalytic cracking, primarily for conversion of C4 and C5 olefins to propylene via auto-metathesis.
In typical olefin plants, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene and C2 acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds ranging in carbon number from C3 to C6. This mixture is separated into different carbon numbers typically by fractionation.
The C3 cut, primarily propylene, is removed as product and is ultimately used for the production of polypropylene or for chemical synthesis such as propylene oxide, cumene, or acrylonitrile. The methyl acetylene and propadiene (MAPD) impurities must be removed either by fractionation or hydrogenation. Hydrogenation is preferred since some of these highly unsaturated C3 compounds end up as propylene thereby increasing the yield.
The C4 cut consisting of C4 acetylenes, butadiene, iso and normal butenes, and iso and normal butane can be processed in many ways. A typical steam cracker C4 cut contains the following components in weight %:
Conventionally, it is common for some of the products of the stream to be separated and the balance recycled back to the olefins unit for pyrolysis or sent offsite as an olefinnic product. The C4 acetylenes are first removed by selective hydrogenation followed by butadiene extraction. Alternately they are hydrogenated along with butadiene to form butenes. Isobutene can be removed by fractionation, by reaction to methyl tertiary butyl ether using methanol, or by reaction with itself and normal butenes in a catalytic C4 dimerization unit. If the stream is to be recycle cracked, the butenes are further hydrogenated to butanes. An alternative processing option is metathesis. As practiced commercially in several units, conventional metathesis involves the reaction of normal butenes with ethylene to form propylene. The isobutene is typically removed before metathesis with ethylene. Isobutene does not react with ethylene or 2 butene under metathesis conditions. Thus isobutene will build up in the system as the C4 fraction is recycled to obtain higher conversions. Isobutylene does however react with product propylene to form ethylene and 2 methyl-2-butene. In many cases this is not desired since it reduces propylene production. Typically after butadiene hydrogenation to normal butenes, over 50% of this stream is linear olefins.
The bottoms from the isobutene fractionation containing primarily the 1-butene and 2-butene are mixed with excess ethylene and passed through the metathesis or olefin conversion reacting step. In this conversion reaction step, the primary reaction is:
2-butene+ethylenexe2x86x922 propylene
The unconverted butenes from the reaction are recycled to obtain a net high conversion of the butenes to propylene.
Typical molar ratios of ethylene/butenes are 1.5 or higher for metathesis with ethylene. Excess ethylene reduces the potential for the butenes to react with themselves thereby reducing the selectivity for propylene formation. The theoretical minimum ethylene required for maximum propylene is 1 mol/mol of 2-butene. The high concentrations of ethylene minimize the non-selective, in terms of propylene, reactions of the butenes with themselves by auto-metathesis. These reactions are shown below:
1-butene+2-butenexe2x86x92propylene+2 pentene
1-butene+1-butenexe2x86x92ethylene+3 hexene
2-butene+2-butenexe2x86x92no reaction
As can be seen, instead of 1 mol of butenes forming 1 mol of propylene and 1 mol of ethylene forming the other mol of propylene, in these auto-metathesis reactions, 2 mols of butene form less than 1 mol of propylene. In spite of the lower selectivity to propylene, this may be an economically desirable route dependent upon the relative values of feeds and products since ethylene is historically higher valued than propylene or butenes. Note however, when the metathesis reaction utilizes ethylene as a co-feedstock, the product of the C5 and C6 normal olefins are reduced.
The C5 and heavier stream from the steam cracker is typically used in the production of gasoline but sometimes the C5""s are separated and recycled to the cracking heaters. A typical steam cracker C5 stream contains the following components in weight %:
This C5 stream contains considerably lower amounts of linear components than the C4 stream. After n-pentadiene hydrogenation, only about 20% of this stream is linear olefins. If the n-pentenes are processed through metathesis, the reactions are:
2-pentene+ethylenexe2x86x92propylene+1-butene
1-pentene+ethylenexe2x86x92no reaction
The C5 stream and the C6 stream are conventionally sent as a bottom product from a fractionation tower to gasoline. In some cases, after hydrogenation, the C5 stream separated by fractionation and is recycled back to the cracking heaters. The C6+ stream after C5 separation is typically sent to gasoline blending since it contains higher octane value aromatics such as benzene in addition to non-aromatic compounds.
For metathesis reactions, the catalyst is typically an oxide of Group VI B or Group VII B metals supported on either alumina or silica supports. In some cases, this oxide is physically admixed with a double bond isomerization catalyst such as MgO. In the reactor, the 2-butene and ethylene are metathesised to propylene. The 1-butene does not react with ethylene. The isomerization catalytic activity incorporated allows 1-butene to be isomerized to 2-butene which is then reacted with the ethylene. The effluent containing propylene, unreacted ethylene and butenes and some C5 and heavier products is first passed through a deethylenizer for removal of that unreacted ethylene and then to a depropylenizer where product propylene is removed overhead. The bottoms may be sent to a debutylenizer where unreacted C4s are recovered and recycled. The C5 and heavier fraction is typically sent to gasoline blending. Alternately, a C4 stream is withdrawn from the depropyleneizer above the bottoms and recycled with the net bottoms of C5 and heavier again being sent to gasoline blending.
In the conventional process for the metathesis of butenes to propylene such as generally described above, there are several problems or disadvantages. First, the reaction takes place with ethylene which not only consumes a valuable olefin but requires recovery for the excess using energy intensive refrigeration systems and then recirculation requiring compression. Secondly, to prepare the feed, there is a separate fixed bed hydrogenation units for butadiene. In the butadiene hydrogenation step, if high 2-butene concentrations are desired, additional hydrogenation is specified in order to maximize the hydroisomerization of 1-butene to 2-butene. High 2-butene concentration is desired because the reaction of 1-butene with ethylene will not occur and thus the 1-butene must be isomerized to 2-butene within the reaction bed itself by a double bond isomerization catalyst such as MgO. In the hydroisomerization of 1-butene to 2-butene in the selective butadiene hydrogenation unit, there is a substantial loss (10+%) of butenes to paraffins due to the added hydrogen which represents a considerable feed loss to the metathesis conversion step. Further, if fractionation is employed for the isobutene removal step, there is an additional loss of butenes since 1-butene is difficult to separate from isobutene without a very expensive fractionation tower.
In the prior U.S. patent application Ser. No. 09/769,871 filed Jan. 25, 2001, an improved process is disclosed and claimed for the processing of the C3 to C6 cut from a cracking process to produce an essentially pure 2-butene stream for the feed to the metathesis reaction process for reaction with ethylene. That improved process involves the use of a catalytic hydroisomerization de-isobutyleneizer tower. In that prior patent application, the metathesis is the typical reaction of 2-butene and ethylene to produce propylene.
Although the yield of propylene is relatively high when utilizing excess ethylene as a reactant, the production of propylene from the cracking cut without the use of ethylene would be desirable, such as when the supply of ethylene is tight and/or ethylene is expensive, even though the selectivity of butenes to propylene is dramatically reduced as long as the increased other products can be used advantageously.
As a part of the background of the present invention, several prior patents are relevant. The Schwab et al U.S. Pat. No. 6,166,279 discloses a process for producing propylene from cracked C4 streams using a two-step process. The first step uses the reaction of 1-butene with 2-butene to form propylene and 2-pentene. In a separate reaction step, 2-pentene is reacted with ethylene to form additional propylene and 1-butene. The 1-butene formed is then isomerized in a third reaction step and recycled to the first reactor as an isomerization mixture of 1 and 2-butene. On a purely theoretical basis, the reactions are:
1 butene+2-butenexe2x86x92propylene+2-pentene xe2x80x83xe2x80x83step 1:
2-pentene+ethylenexe2x86x92propylene+1-butene xe2x80x83xe2x80x83step 2:
The net reaction of these two steps is:
2-butene+ethylenexe2x86x922 propylene
This is identical to the base metathesis reaction. The preferred feed mixture is a mix of 1-butene and 2-butene where the 1-butene is in excess. This is achieved by choice of feedstock composition and by recycling the 1-butene produced in step 2. Under these conditions, some reaction between two 1-butene molecules will result in the formation of ethylene and 3-hexene. This formation of ethylene from butenes shifts the overall selectivity of the net reaction such that on a fresh feed basis, less ethylene and more butenes are required per unit of propylene.
U.S. Patent Application Publication US2001/0003140 A1 discloses separately the second step above, namely the reaction of 2-pentene with ethylene to form propylene and 1-butene. Similarly, U.S. Pat. No. 5,698,760 discloses a process where a mixed pentene stream is reacted with ethylene under metathesis conditions to form butenes and propylene. U.S. Pat. No. 6,159,433 and U.S. Pat. No. 6,075,173 disclose processes for reacting steam cracker C4""s consisting of reacting the butenes streams with ethylene to form primarily propylene.
U.S. Pat. No. 5,043,520 discloses a process where olefins ranging from C2 to C100 are contacted with a metathesis catalyst physically admixed with an acidic zeolitic double bond isomerization catalyst. The concept of using a physically admixed double bond isomerization catalyst has been well known. In the preprints of the Symposium on Hydrocarbon Chemistry, Division of Petroleum Chemistry, September, 1972 American Chemical Society meeting, R. L. Banks of Phillips Petroleum states, xe2x80x9cHigh selectivity to primary disproportionation products is desirable for many applications and this can be achieved by reducing double bond isomerization activity of catalysts. However, for certain applications, such as processing detergent range linear olefins from propylene, high double bond activity is essential; symmetrical olefins such as 2-butene produced from the disproportionation of propylene, will not disproportionate and a shift in location of the double-bond is needed prior to the disproportionation reaction. Incorporation of acid-type double bond isomerization catalysts in the system would also promote skeletal isomerization and dimerization, resulting in branched products. Magnesium oxide is also a very selective catalyst for double bond isomerization and is compatible with tungsten oxide catalyst.xe2x80x9d
Alpha olefins are important co-monomers in the production of both polyethylene and polypropylene. In U.S. patent application Ser. No. 09/863,973, which is incorporated herein by reference, a process for producing a catalyst and a process for the isomerization of internal olefins to alpha olefins is disclosed. In one example, a mixed n-butenes stream consisting of 1-butene and 2-butene after removal of isobutene is passed through a combined isomerization/fractionation step to produce essentially pure 1-butene as an overhead product from the fractionator and a bottoms stream consisting of essentially pure 2-butene. The 2-butene stream can either be sent to product or recycled through the isomerization step to form more 1-butene. Similarly 3-hexene can be isomerized and fractionated to produce 1-hexene.
The separation of closely boiling olefin isomers is quite difficult. This is usually done in super-fractionators employing many fractionation stages and extremely high reflux ratios. Further, even at high temperatures, the equilibrium concentration of the alpha olefin is low compared to the other isomers. For a mixed C4 stream, at 650xc2x0 F. reaction temperature, the 1-butene content at equilibrium is 22% with the balance being 2-butene. For the hexene stream, the concentration of 1-hexene at 650xc2x0 F. is 8% with the balance being 2 and 3-hexene. In a process to isomerize and then fractionate a mixed olefin stream to recover high purity alpha olefins, the relative volatility between isomers is very close such that high reflux ratios and large number of separation stages are required. Also with the feed mixture at low concentration, high recycle through the isomerization section is required which increases the tower cost and energy requirements even further. If an alternate route could be achieved that avoided the extensive recycle and super-fractionation for the production of alpha olefins, there would be considerable economic benefit.
The object of the present invention is to provide an improved process for the conversion of olefins for the production of propylene from a C4 cut from a steam or other cracking process. The invention involves the auto-metathesis of a mixed normal butenes feed in the presence of a metathesis catalyst and specifically operates without any ethylene in the feed mix to the C4 metathesis unit. Some fraction of the 2-butene feed may be isomerized to 1-butene and the 1-butene formed plus the 1-butene in the feed react rapidly with the 2-butene to form propylene and 2-pentene. The feed to the reactor also includes the recycle of the 2-pentene formed in the reactor with unreacted butenes to simultaneously form additional propylene and hexene. In one embodiment, some or all of the 3-hexene formed in the reaction is isomerized to 1-hexene. In another embodiment, some portion of the 3-hexene produced in the main metathesis reaction is reacted with ethylene to produce 1-butene without the need for super-fractionation. In another embodiment, the 3-hexene product is hydrogenated and recycled back to the cracking heaters.
In a further embodiment, the preparation of the feed for the metathesis reaction from steam cracker C4""s involves a system using catalytic distillation hydrogenation to maximize the 2-butene content while simultaneously removing the isobutylene in the C4 stream.